Gas chromatography (GC) is a well known analytical technique where gas phase mixtures are separated into their individual components and subsequently identified. The technique may be employed to obtain both qualitative and quantitative information about the components of the mixture. Specifically, the separation mechanism employs two different media, one moving (mobile phase) and one unmoving (stationary phase). In GC, the mobile phase is normally hydrogen or helium, which flows across the stationary phase, which is a solid or otherwise immobilized liquid on a solid support or the interior capillary wall.
The sample mixture is introduced into the mobile phase stream and the residence time of each component of the sample in the stationary phase relates to differences in their individual partitioning constants with respect to the two solvent phases.
Samples for GC are usually liquid and must be volatized prior to introduction to the mobile phase gas stream. GC analysis is typically divided into four stages:                1. sample preparation, where liquid samples are heated and volatilized,        2. sample introduction, where the sample vapor is loaded all or in part onto the analytical column,        3. separation, where the sample is separated into its individual components as it passes through the analytical column, and        4. detection, where the separated components are identified as they exit the analytical column.        
In conventional GC instrumentation the first two steps are achieved in the sample inlet hardware. Inlet hardware often includes a replaceable sleeve, or liner. Liners are normally operated at elevated temperatures, e.g., over 200° C. This enhances the rate of sample vaporization and reduces adsorption on the inner surface of the liner [1]. Many internal configurations are available for liners, as well as coatings for them [2-12].
In most cases the configuration serves to enhance the degree of sample volatilization from the point of exit from the syringe needle to the column entrance, and provide gas phase sample homogeneity from components within the liquid mixture having different boiling points. A simple configuration for an inlet liner is a straight cylindrical tube of glass having a consistent inner diameter along the longitudinal path. Other configurations include more complex inner paths intended to increase turbulence, affect the comparatively short residence time the liquid sample is in the liner, or interrupt the liquid stream leaving the syringe needle. These internal configurations include tapers or goosenecks, baffles, funnels, inverted cup elements, spiral regions, points of flow constriction, and drilled holes along the longitudinal path of the liner.
Other elements of liners optionally include small plugs of packing materials such as glass wool [1], Carbofrit™ (Trademark of Restek Corporation) packing material, and more recently inert metal wire bundles [13] which serve as additional surface area sources for heat transfer into the sample and as a physical filter for any solid/nonvolatile contaminants present in the liquid sample.
During the injection process, it is also important to minimize sample/liner and (if applicable) sample/packing material interactions that can result in undesirable chemical reactions, decomposition, or permanent adsorption of the sample. It is equally important that the liner not contribute contaminants to the analysis, which may result in spurious peaks or an increase in the baseline signal in detection measurements of the components contained in the sample being analyzed.
In cases where liners become contaminated with solid/nonvolatile species it is necessary to replace them from time to time. Liner replacement frequency depends on the type of sample; with ‘dirty’ samples having high concentrations of high boiling point components or large amounts of nonvolatile particulate matrix shortening the service life of the liner.
Liners are manufactured from glass, primarily borosilicate, but also fused quartz, and less commonly from metal, mainly stainless steel [14].
Because of the techniques commonly employed in using a liner in a GC instrument it is often desirable for the liner to be transparent. It is particularly important to be able to see through the walls of these liners which contain packing material in order to ensure its proper plug position within the internal bore of the liner. It is also advantageous to be able to observe wool placement, and to be able to check for the presence of debris or other visual contaminants.
Various chemical coatings are applied to liners in order to reduce the degree of interaction between the sample and the surface of the liner. Sample-surface interactions may result in sample absorption in the coatings, decomposition of the coatings, and formation of new reaction products; in each case resulting in undesirable peaks (or loss of desirable ones) in detection measurements of the components contained in the sample being analyzed in the separation analysis. In addition to low sample-surface interactions, it is also desirable for the liner coating to be thermally stable in order to minimize background signal contributions originating from the liner coating itself detected by the analytical equipment. For glass substrate liners, common deactivation techniques include chemically treating the exposed silanol groups with organosilane reagents such as hexamethyldisilazane (HMDS), dimethyldichlorosilane (DMCS), and trimethylchlorosilane (TMCS) [15].
Another suitable coating is vapor deposited silicon, including the Siltek® Sulfinert® coatings [15-19]. This coating has been demonstrated on both borosilicate glass and metal surfaces, in both cases resulting in an opaque mirror finish whose color is dependant on the coating thickness.
Distinguishing liners from one supplier to another is largely dependant on printed markings or logos on the outer surface. Identifier marks have also been employed to describe liner orientation, lot number, part number, wool placement, etc. Common glass marking techniques include silk screen printing of enamels that are then baked onto the surface, controlled etching, or combinations of the two. The latter method suffers from poor contrast and hence visibility of the image is impaired, while the former introduces a chemically new surface with the potential of deleterious interactions with analytes. Deactivation processes optimized for silanol or glass surfaces can also yield bare areas or weakly bonded moieties that can exude contaminating volatiles into the flow stream at elevated temperatures. To be useful, suitable manufacturer liner identifiers in the practice of gas chromatography must be resistant to visual fading resulting from cycles to high temperature, and not interfere with the analysis, particularly by contributing gas phase contaminants into the sample analysis path.
One unique marking case is the Sulfinert™ (Trademark of Restek Corporation) process, which results in a fully deactivated and distinctive rainbow colored mirror finish which is readily recognizable, even at a distance
During normal operation of GC instrumentation it is necessary to remove or replace one liner for another. Liners are routinely heated to over 200° C. during the sample analysis and may be removed and/or replaced between sample analysis runs. While GC instrument companies recommend first waiting for the instrument to completely cool, many users do not wait and remove liners from the system while they are still hot. This creates a safety risk for the operator because current liners are visually indistinguishable between hot or cooled states. This risk relates to all types of liners, both metal and glass.
Another weakness of the current technology relates to liners that have been previously used or subjected to high temperature. In many cases the deactivation coating may be compromised through excessive use or extended exposure of the liner to oxygen while hot. In these cases, the degree to which the coating contributes to the background signal of the analysis or loss of analytes due to adsorption or degradation is increased. In general, liners that have been used for prolonged periods behave more poorly than do new liners. In addition, dirty samples may leave residue behind in the liner that may impact future runs.
In some cases, sample residue may be visible as dark colored stains in the liner. In others, liners with compromised surfaces may appear clear and colorless. In addition, indications of residue are obscured while the liner is installed in the GC.
Discussion on Porous Glass
Two common methods of producing porous glasses are through sol-gel routes and from phase separation of high boron oxide containing borosilicate glasses, followed by acidic leaching. The latter method, commonly known as the Vycor process (Vycor is a trademark of Corning Glass Works) [20] allows for convenient fabrication of complex shapes using standard glass manufacturing equipment, and gives porous products of the same dimensions comprised of approximately 96% silicon dioxide when subjected to aqueous acid leaching. The pores can be blocked through high temperature sintering under vacuum with a significant amount of physical shrinkage, or by chemical means through internal deposition of polymers or other blocking agents. The polymers can be organic, inorganic, or combinations thereof. The siloxane-based polymers are of particular interest because of their convenient compatibility with glass surfaces and can be used in the preparation of composites with high thermal stability.
The degree of pore blockage can be tailored from partial to complete, depending on the nature of the chemistry and processes employed. For example, partial pore blocking with silicon dioxide precursors such as tetrachlorosilane, tetraethoxysilane and tetramethoxysilane can yield a channel structure that is highly selective toward hydrogen permeation over other gases of larger physical molecular size. [21-24].
Discussion on Doping and Dopants
Because the channels in porous Vycor and sol-gel produced glass can range from less than ten to hundreds of angstroms in diameter, they can be filled or doped with a wide variety of organic and inorganic materials. For example, small iron oxide particles can be synthesized within the pores through infusion of aqueous ferric nitrate solutions followed by thermal decomposition. Further modifications of the aggregates can be made by heating in a hydrogen atmosphere. [25] The restricted geometry inside the confined space within the pore can affect the spectral absorptive and fluorescence properties of the dopant chromophore [26]. See also [27]. Other metal ions and mixtures have also been used as dopants in porous glasses. [28-30].
Organic and organometallic compounds with useful optical and electronic properties have been used as dopants in porous Vycor glass [31-33].
Organic dyes have been infused into Vycor pores [34-36], Unblocked pores containing dopants have been used in gas sensing applications [37]. The dopants have also been encapsulated by organic polymers within these pores[38-42].
The mild conditions required for the creation of sol-gel glasses is compatible with a wide range of dopants that constitute the same family as those described above for infusion into porous Vycor glass. Because of a lack of interconnected pores, the chromophoric species are usually present during the gelation process. Further ripening of the gel can take place without damaging the entrained dyes, salts, or chromophoric particles [43-49].
Discussion on Thermochromic and Photochromic Dyes
Dyes that produce a color change with temperature are referred to as thermochromic and have been the subject of extensive study [50-56]. A number of chemical classes exhibiting this behavior have been reported, and include perylene dyes, encapsulated leucodyes, and some inorganic compounds. In addition, many photochromic compounds (those that change color on exposure to light at specific wavelengths) exhibit thermochromic behavior. Some examples of this class of compounds include spiropyrans, spiroxazines, and ethylene aromatics, and the color change is due to often reversible molecular rearrangements giving rise to differences in conjugation or ionic structure. Thermochromic and photochromic indicators have been introduced into both porous Vycor glass, and sol-gel glasses, with the polarity and geometric constraints in their vicinity affecting the stabilization of one isomeric form over another. Thus environmental effects can result in thermochromic or reverse thermochromic behavior with the same dye molecule. Encapsulation of the dyes within the pores of these inorganic glass matrices has been reported using polymethylmethacrylate to yield transparent composites with the anticipation that they could lead to the development of three-dimensional high density memory arrays [47].
The thermochromic effect can be gradual, with the color intensity varying with the percentage of dye molecules in each state. Photochromic images previously set through light exposure can sometimes be erased through thermochromic relaxation.